Aluminum base alloys and process for obtaining same



Jan. 20, 1970 wm-r R ET AL 3,490,955

ALUMINUM BASE ALLOYS AND PROCESS FOR OBTAINING SAME Filed Jan. 23, 1967INVENTORS.

JOSEPH WINTE'R ALAN J. GOLDMAN WILL/AM C. SETZER A TTO/PNE V UnitedStates Patent U.S. Cl. 148-115 Claims ABSTRACT OF THE DISCLOSURE Thepresent invention relates to aluminum base alloys having high strengthprepared by working at a temperature below 450 F., holding at from 250to 650 F. and working again at a temperature below 450 F.

The present invention relates to a process for the preparation of highstrength aluminum base alloys. In particular the present inventionresides in a process, an an alloy produced thereby, for the preparationof alumlnum base alloys having strengths considerably higher thanconventionally, even with the introduction of severe amounts of coldwork.

It is naturally highly desirable to conveniently obtain optimum highstrengths in aluminum base alloys, especially in those common,inexpensive, commercially available aluminum base alloys. Variousprocesses are generally known for increasing the strengths of aluminumbase alloys. Frequently these processes are expensive and cumbersome orcharacterized by a plurality of process steps inconvenient and expensiveto utilize. In addition, conventional processes are frequentlycharacterized by critically defined process conditions which makes theprocess inconvenient to operate on a commercial scale. Furtherm re,processes for increasing the strength of aluminum base alloys arefrequently selective based on particular alloying ingredients present inthe alloy and are not often utilizable over a wide range of aluminumbase alloys.

In addition to the foregoing, processes for increasing the strength ofaluminum base alloys still frequently leave much to be desired withrespect to the ultimate strength obtained. In addition, conventionalprocesses often increase the strength of the aluminum base alloy withattendant losses'of other desirable physical properties, thereby oftenimproving one property with an attendant degradation of another.

Three standard ways to increase physical properties are by alloying,cold working, and second phase precipitation effects. Each of theseadversely affects some other mechanical or physical property. Forexample: alloying is invariably associated with decrease inconductivity; cold working decreases the ductility or elongation; andprecipitation hardening will decrease toughness and increase notchsensitivity and decrease corrosion resistance. Also, each of thesegenerally has other deleterious effects.

It is, therefore, an object of the present invention to provide aprocess for preparing aluminum base alloys having improved strengthcharacteristics.

It is an additional object of the present invention to provide animproved alloy and process as aforesaid which is inexpensive andconvenient and readily feasible on a commercial scale.

It is a still further object of the present invention to provide animproved alloy and process as aforesaid which attains greatly improvedstrength characteristics without inordinate loss of desirable physicalproperties, for example, electrical properties and finishingcharacteristics.

3,490,955 Patented Jan. 20, 1970 Additional objects and advantages ofthe present invention will appear hereinafter.

In accordance with the present invention, it has now been found that theforegoing objects and advantages may be readily attained and an improvedalloy and process conveniently provided.

The process of the present invention comprises:

(A) Providing an aluminum base alloy containing from 0.05 to 1.0% iron,from 0.05 to 1.0% silicon, at least one material selected from the groupconsisting of less than 10.0% magnesium, less than 3.0% manganese, lessthan 1.0% copper, less than 0.5% chromium, less than 0.5 zinc, less than0.5 zirconium, less than 0.5 titanium, less than 0.1% boron, others lessthan 0.5 each, total less than 1.5%, balance essentially aluminum;

(B) Working said alloy, preferably by rolling or drawing, at atemperature below 450 F., with a total reduction in excess of 20%;

(C) Holding said alloy at a temperature of from 250 to 650 F. for aperiod of time no greater than defined in the following formula: T(8.95+log t)=5,700, wherein T is temperature in degrees Kelvin and t isthe maximum time in minutes at temperature T, so that there is norecrystallization throughout the matrix and so that there is less than10% loss in yield and tensile strength; and

(D) Repeating step (B), preferably repeating steps (B) and (C),preferably a plurality of times.

In accordance with the present invention it has been found that theforegoing process results in a surprising improvement in strengths, evenin the common aluminum alloys, and even with the introduction of thermaltreatments after severe amounts of cold working. For example,

tensile properties have been reproducibly obtained in excess of 53,000p.s.i. for aluminum alloy 3003, in excess of 45,000 p.s.i. for aluminumalloy 5005, in exess of 35,000 p.s.i. for aluminum alloy 1100 and inexcess of 35,000 p.s.i. for EC grade aluminum. Throughout the presentspecification, aluminum alloy numbers represent Aluminum Associationdesignations. This is particularly surprising since normally thermaltreatments after cold working result in a considerable decrease in yieldand tensile strengths in order to increase ductility.

In general, the present invention is broadly applicable to a wide rangeof aluminum base alloys as stated above, including high purity aluminum,and significant improvement is obtained with all these materials. It ispreferred, however, that the aluminum base alloy contain less than 99.5%aluminum and naturally that certain additional elements be present inthe alloy. This is reflected in the following which shows thepermissible and preferred amounts of additional elements wherein allpercentages are percentages by weight: Silicon from 0.05 to 1.0%,preferably from 0.3 to 0.7%; iron from 0.05 to 1.0%, preferably from 0.4to 0.8%. In addition to iron and silicon, the alloy must contain atleast one of the following materials: copper from 0 to 1.0%, preferablyfrom 0.1 to 0.3%; manganese from 0 to 3.0%, preferably from 0 to 1.6%;magnesium from 0 to 10.0%, preferably from 0 to 5.0%; chromium from 0 to0.5 preferably from 0 to 0.2%; zinc from 0 to 0.5 preferably from 0 to0.3%; zirconium from 0 to 0.5 preferably 0 to 0.3%; boron from 0 to0.1%, preferably from 0 to 0.05%; titanium from 0 to 0.5 preferably from0 to 0.2%, others each less than 0.5%, total less than 1.5%, preferablyeach less than 0.05%, total less than 0.15%. Particularly preferredalloys include aluminum alloy 5005, 3003, 1100, EC grade aluminum,superpurity aluminum, etc. In general, the preferred alloys are those ofthe 1000 series, 3000 series and 5000 series.

In accordance with the present invention the aluminum base alloys may becast in any desired manner. The particular method of casting is notcritical and any commercial method may be employed, such as Direct Chillor Tilt Mold casting. The alloys may also be hot rolled to plate form ina conventional manner.

After casting it is preferred in accordance with the present inventionto provide a homogenization or solutionizing treatment. Thissolutionizing treatment should be performed at a temperature above 850F. and preferably above 950 'F. and the ingot should be held attemperature for a minimum of 4 hours. After the solutionizing step, theingot should be rapidly cooled to below 450 F. and preferably rapidlycooled to below 250 F. at a rate of above 400 F. per hour.

In accordance with the present invention, if desired, the solutionizingstep may be combined with the casting operation, i.e., in the castingoperation the material may be held at the requisite temperature for therequisite period of time followed by rapidly cooling.

The purpose of the solutionizing step is as follows. When the aluminumbase alloy contains alloying additions as indicated hereinabove, thesolutionizing step followed by rapid cooling puts as much of these materials into solution as possible. Thus, the solute elements or alloyingadditions are in solid solution, preferably to the maximum degree, inthe aluminum or solvent matrix. This is, as stated hereinabove, apreferred operation.

In accordance with the present invention, the next step is the criticalworking operation. The preferred type of working is naturally rollingand the present specification will be particularly directed to this formof initial working. It should be understood, however, that other typesof working are contemplated, especially in later steps, such as drawing,swaging, etc.

The material is worked or rolled at a temperature below 450 F. with atotal reduction in excess of 20%. It is preferred to roll at atemperature below 200 F. The material may be rolled in one or morepasses with the amount of reduction per pass not being critical. Ingeneral, it is preferred to take a plurality of smaller reductionsrather than one large reduction. In general, each pass should take atleast a 15% reduction. Large reductions may be taken, if desired, forexample, reductions in excess of 99% may be taken, e.g., in wire form.Throughout the present specification, the term reduction means totalreduction in area.

After the rolling or working step the material is critically held atfrom 250 to 650 F. for a period of time no greater than defined in thefollowing formula:

T (8.95-I-log t) =5,700

wherein T is any given temperature within the foregoing temperaturerange in degrees Kelvin and t is the maximum time in minutes attemperature T. The minimum time at temperature is not particularlycritical, but should be at least one second. Naturally, the higher thetemperature within the foregoing temperature range, the shorter themaximum holding time and the lower the temperature the longer themaximum holding time. It is preferred to operate in the temperaturerange of from 250 to 450 F. Examples of maximum allowable timesdetermined in accordance with the foregoing formula are: approximately400 hours at 300 F.; approximately 16 hours at 400 F.; and 2 minutes at650 F.

As indicated above, after the rolling or working step the material iscritically held at from 250 F. to 650 F. for no longer than the timedetermined by the foregoing empirical equation for which the constantswere determined experimentally. It is interesting to note that changingthe form of this equation to l/t=exp (Q/RT) gives a value of Q, theactivation energy, that is slightly lower than is required forrecrystallization in aluminum. This indicates that the initiation ofrecrystallization is the upper limit for the thermal treatment.

Subsequent to the'thermal treatment, the material is worked or rolledagain at a temperature below 450 F. with a total reduction of at least20% in the same manner as indicated hereinabove. This second rolling orworking step may be the final step, or may, and preferably is,

' then followed by an additional thermal treatment at from 250 to 650 F.as indicated hereinabove.

Cold working after a low temperature thermal treatment is unusual in thefabrication of wrought aluminum structures inasmuch as low temperaturetreatment or partial annealing are normally introduced to stabilize thestructure or lower the strength to desired levels in order to meetspecific properties. In fact, the H2X and H3X standards of the AluminumAssociation specifies work hardening and partial annealing or workhardening and then stabilizing. In accordance with the presentinvention, however, a stabilizing or partial annealing as a preparatorystep for subsequent cold working provides the significant mechanicalproperty increase of the present invention.

It is preferred to repeat the rolling and thermal treatment steps aplurality of times, preferably from 3 to 5 times. In accordance with thepresent invention, the final step in the process may be either therolling or working step or the thermal treatment step upon particularrequirements.

A modification of the present invention includes the following. Ifdesired, the rolling step may be performed within the thermal treatmentrange. Thus, where one rolls at a temperature of from 250 to 450 F. andholds the material at temperature one may efi'etcively combine theworking or rolling step with the thermal treatment step and therebyavoid a separate thermal treatment step.

An additional modification includes the following: The final step mayoptionally be the holding step of the present invention at from 250 to650 F. but for a longer period of time than permitted by the foregoingformula, so that there is no recrystallization throughout the matrix butthere is less than 25% loss in yield and tensile strength. This wouldresult in yield and tensile strengths still greatly superior thannormally obtained, but the ductility would be increased.

In accordance with the present invention the first rolling operation orthe first deformation forms a cellular sub-grain structure. That is, themicrostructure of the alloy is characterized by grains within grains.The thermal treatment step tends to stabilize the sub-grain walls bymigrating solute atoms towards the sub-grain walls. The seconddeformation forms more sub-grain walls within the sub-grain structure,thereby incrementally refining the sub-grain size finer and finer asdeformation and thermal treatment steps are repeated.

Thus, the improved alloys of the present invention are characterized bygreatly improved strength characteristics and ultra fine sub-grainstructure with the sub-grain.

EXAMPLE I In the following examples the following alloys were used.Aluminum alloy 1100; aluminum alloy 3003; aluminum alloy 5005; andsuper-purity aluminum. All of the alloys were Direct Chill cast andscalped into ingots 1% x 4 x 6".

EXAMPLE 11 In this example Alloy 3003 as cast was cold rolledincrementally from 1.750 to 1.5 to 1.25" to 1.0" to 0.8" to 0.65" to0.5" to 0.35 to 0.25 to 0.175" to 0.122" to 0.083" to 0.07" to 0.05" to0.036" to 0.025 to 0.018 to 0.014. After each reduction except the lastthere was a minute holding step at 400 F. The resulting mate rial had anaverage yield strength at 0.2% offset of 58,- 200 p.s.i. and an ultimatetensile strength of 63,400 p.s.i. with 2% elongation.

For comparative purposes Alloy 3003 was cold rolled to 0.014" gage andhad the following properties: yield strength 0.2% offset of 27,000p.s.i.; ultimate tensile I strength of 31,000 p.s.i. with 2% elongation.

In accordance with the present invention, FIGURE 1 is a photomicrographof aluminum alloy 3003 obtained in accordance with the foregoing exampleat 0.036 gage. FIGURE 2 is a photomicrograph of aluminum alloy 3003 at0.036" gage, with the alloy prepared in the following manner; the alloywas homogenized at 1100 F., hot rolled starting 950 F. and cold rolledto gage. Both photomicrographs are at a magnification of 30,000 Thephotomicrographs were prepared by a transmission electron micrographtaken from thin foils prepared by electro-chemical milling of a coldrolled material to a thickness of approximately 2,000 Angstroms.

From examination of the photomicrographs, it can be seen that FIGURE 2has gross areas of dislocation tangles interspaced with large areas ofapparently unworked materials. On the other hand, FIGURE 1, the alloy ofthe present invention has a series of recognizable, discrete grains ofapproximately 0.0001 mm. No regions of apparent unworked material arevisible. The discrete subgrains are separated by recognizable grainboundary walls.

EXAMPLE III Aluminum alloy 3003 prepared in Example I was heated to 1100F. and held for 16 hours. It was then water quenched to room temperaturein 5 seconds followed by cold rolling incrementally from 1.75" to 1.5"to 1.25" to 0.8 to 0.65" to 0.5" to 0.35" to 0.25" to 0.175" to 0.122 to0.088". Following this, the material was further cold rolledincrementally, except that after each reduction except the last thematerial was heated to 400 F. and held for 10 minutes at temperature andwater quenched to room temperature. The reductions were as follows: from0.088" to 0.072 to 0.05" to 0.036" to 0.029" to 0.024" to 0.020" to0.017" to 0.013". The resultant material had an average yield strengthof 48,000 p.s.i. at 0.2% offset and an ultimate tensile strength of55,400 p.s.i. with 2% elongation. The microstructure was similar to thatshown in FIGURE 1. Identical material processed in the same mannerwithout the interanneals had a yield strength of only 38,000 p.s.i. at2% offset and an ultimate tensile strength of 43,300 p.s.i. with 4%elongation.

EXAMPLE IV Aluminum alloy 5005 prepared in Example I was cold rolledincrementally from 1.75" to 1.5" to 1.25" to 1.0 to 0.8" to 0.65" to0.5" to 0.35 to 0.25" to 0.175" to 0.122" to 0.085" to 0.06" to 0.042"to 0.03 to 0.022". After the last cold reduction the material was heldfor 10 minutes at 300 F. followed by an additional cold reduction to0.018". The resultant material had an average yield strength of 48,900p.s.i. at 0.02% offset and an ulti mate tensile strength of 49,800p.s.i. with an elongation of 1%. As a comparison, aluminum alloy 5005was cold rolled to 0.018" gage and had the following properties: yieldstrength 28,000 p.s.i. at 0.2% offset; ultimate tensile strength 29,000p.s.i.; with an elongation of 1%.

What is claimed is:

1. A process for preparing high strength wrought aluminum base alloyswhich comprises:

.(A) providing an aluminum base alloy consisting essentially of from0.05 to 1.0% iron, from 0.5 to 1.0% silicon, at least one materialselected from the group consisting of less than 10.0% magnesium, lessthan 3.0 manganese, less than 1.0% copper, less than 0.5% chromium, lessthan 0.5% zinc, less than 0.5% zirconium, less than 0.5% titanium, lessthan 0.1% boron, balance essentially aluminum;

(B) working said alloy at a temperature below 450 F. with a totalreduction in excess of 20%;

(C) holding said alloy at a temperature of from 250 to 650 F. for aperiod of time of at least one second but no greater than defined in thefollowing formula: T (8.95 +log t)=5,700, wherein T is temperature indegrees Kelvin and t is the maximum time in minutes at. temperature T,so that there is no recrystallization throughout the matrix and so thatthere is less than 10% loss in yield and tensile strength; and

(D) repeating step (B).

21A process according to claim 1 wherein steps (B) and (C) are repeated.

3. A process according to claim 1 wherein steps (B) and (C) are repeatedin a plurality of times.

4. A process according to claim 1 wherein the materials in step (A) arepresent in the following amounts: silicon from 0.3 to 0.7%, iron from0.4 to 0.8%, at least one material selected from the group consisting ofcopper from 0.1 to 0.3%, manganese up to 1.6%, magnesium up to 5.0%,chromium up to 0.2%, zinc up to 0.3%, titanium up to 0.2%, zirconium upto 0.3% and boron up to 0.05%.

5. A process according to claim 1 wherein prior to said working step-(B) the material is homogenized at a temperature above 850 F. for atleast 4 hours.

6. A process according to claim 5 wherein after said homogenization stepthe material is rapidly cooled to below 450 F.

7. A process according to claim 1 wherein step (B) is rolling at atemperature below 200 F.

8. A high strength wrought aluminum base alloy consisting essentially offrom 0.05 to 1.0% iron, from 0.05 to 1.0% silicon, at least one materialselected from the group consisting of less than 10.0% magnesium, lessthan 3.0% manganese, less than 1.0% copper, less than 0.5% chromium,less than 0.5% zinc, less than 0.5% zirconium, less than 0.5% titanium,less than 0.1% boron, balance essentially aluminum, said alloy havingultra fine subgrain structure with the sub-grain size being less than0.0001 mm., with the sub-grains having boundary walls of pinneddislocation tangles.

9. An alloy according to claim 8 wherein the matrix between dislocationtangles consists of individual regions having lower content of alloyingadditions and low density of dislocations.

10. An alloy according to claim 9 containing from 0.3 to 0.7% silicon,0.4 to 0.8% iron, at least one material selected from the groupconsisting of 0.1 to 0.3% copper, up to 1.6% manganese, up to 5.0magnesium, up to 0.2% chromium, up to 0.3% zinc, up to 0.2% titanium, upto 0.3 zirconium, up to 0.05% boron.

References Cited UNITED STATES PATENTS 2,168,134 8/1939 Pavelka 148-1153,232,796 2/1966 Anderson 148-115 3,366,476 1/1968 Jagaciak 148-115RICHARD O. DEAN, Primary Examiner US. Cl. X.R.

33 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,490,955 Dated January 20, 1970 Inventor(s) Joseph Winter, Alan Goldmanand William C. Setzer It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

I- In Column 1, line 22, the word "an", first occurrence,

should read --and-.

In Column 2, line 36, the word "exess" should read -excess-.

In Column 4, line 25, after the words "treatment step" insert--depending-;

In Column 4, line 31, the word "effetcively" should read--effectively--.

In Column 5, line 25, after the words "milling of" the word "a" shouldread --as--;

In Column 5, line 29, the word "interspaced" should read--interspersed--.

In Column 6, line 2, after the word "from", second occurrence, change"0.5" to read --0.05--;

In Column 6, line 5, after the word "than", first occurrence, change"3.0" to read --3,0%--;

In Column 6, line 23, after the words "are repeated" delete the word"in".

316N223 Alia November 17, 1970 Bdndlflmhqh. mm I. m Mug Offic Oomissiomor

